This invention relates generally to the development of a method for making articles from metal alloys which have predetermined inclusions that are controlled with respect to their size, shape, composition and crystal structure, and also with respect to their location and orientation within the article, such as reference standards. Specifically, it relates to a method for making titanium alloy reference standards having brittle, hard, alpha-phase titanium inclusions which are controlled with respect to the factors described above.
The usage of many metal alloy systems is limited by the presence of certain metallic or intermetallic inclusions which often represent carbide, oxide, nitride, sulfide or other compounds of one or more of the alloy constituents, and which result from natural impurities associated with the alloy constituents, processing of the alloy or other sources. However, such inclusions are very often difficult to detect, and if detectable are difficult to characterize quantitatively using techniques such as ultrasound analysis because of their generally random nature (e.g. size, shape, orientation, location, composition, crystal structure). One example is brittle, "hard-alpha" inclusions found in titanium alloys. This form of titanium is stabilized by locally high concentrations of embrittling elements such as nitrogen and oxygen. Improved ability to detect small, hard-alpha titanium inclusions would permit more efficient designs of high temperature gas turbines and their components. However, naturally-occurring hard-alpha inclusions are rare in commercially produced titanium alloy components. The number of such inclusions has been reported to average about one per million pounds of titanium alloy produced. Also, hard-alpha inclusions are not generally found naturally in suitable forms to permit the development of new detection and inspection techniques, or the calibration of existing analytical equipment, so as to improve the ability or probability of detecting such inclusions, because the development of reliable detection techniques for inclusions such as hard-alpha inclusions typically requires characterization of several of their physical properties over a range of anticipated compositions and morphologies. For ultrasound techniques, this characterization might include the measurement of differential sound velocities, reflectance and density as a function of inclusion composition, size, shape, orientation and location, and then relating such measurements to detectability of the inclusions. Characterization is also often complicated by the fact that thermomechanical processing, such as forging and rolling, tends to crack the frequently brittle inclusions such that the characterization of their detectability is made ambiguous, because the presence of the crack is more detectable, using standard techniques such as ultrasound detection, than the presence of the inclusion.
Prior art methods for making alloys with controlled inclusions have included drilling holes or creating other mechanical discontinuities in articles made from an alloy of interest. However, there are major deficiencies with such methods. Mechanical discontinuities present the largest change possible in the physical characteristics of a material (e.g. for ultrasound measuring techniques, the speed of sound and reflectance) while actual inclusions, such as hard-alpha titanium within a titanium alloy, typically exhibit much less difference in physical characteristics in comparison with the alloy in which it is located. Thus, void-like or crack-like discontinuities are easier to detect than inclusions such that calibrating detection equipment from these discontinuities or inclusions which also contain these discontinuities, may not necessarily yield accurate extrapolations regarding the detectability of such inclusions.